Hans Jenny 109 



Loosjes (28) hinted that Liebig foresaw the modern contact theory 

 of plant nutrition in soils. This contention is hardly justified. In Liebig's 

 days two essential prerequisites of the contact exchange theory were 

 unknown; namely, the law of mass action, especially in relation to its 

 reversibility aspects, and the concept of the electric double layer. More- 

 over, Liebig himself stated (p. 139) : "It is very difficult to visualize in 

 what manner plants contribute to the solubility of mineral constituents; 

 that water is essential to their passage is obvious." 



The idea that the extracted soil solution does not provide the whole 

 answer to the question of mineral nutrition of plants in soils was 

 sporadically advanced by several investigators, such as Comber (7), 

 Kossovitch (25), and Truog (44). These attempts to attribute a more 

 active role to the solid phase were unsuccessful for two main reasons. 

 First, no reliable experimental material was produced which would 

 render untenable the soil solution theory, especially if combined with 

 the picture of carbon dioxide excretions. Second, and most important 

 of all, no theory was advanced which provided a convincing reaction 

 mechanism involving the solid phase. 



SALIENT ASPECTS OF THE CARBON DIOXIDE THEORY 

 OF MINERAL NUTRITION OF PLANTS 



According to prevailing ideas, roots secure cations adsorbed on soil 

 particles by means of carbonic acid exchange (Figure 1). The transfer 

 involves the following steps: 



1. Release of carbon dioxide from the root and formation 

 of carbonic acid. 



2. Diffusion of carbonic acid to the distant clay surface. 



3. At the clay surface H+ replaces K+; the clay particle 

 becomes acid. 



4. The new ion pair, K+HC0 3 "~ , returns to the root sur- 

 face. 



5. At the root surface K+ exchanges for H+, or K+HC0 3 — 

 enters the root as an ion pair. 



Of the last two alternatives the latter seems less probable. Overstreet, 

 Ruben, and Broyer (j6) immersed barley roots in a solution of potas- 



